Recently, image display and illumination devices using electroluminescence light-emitting elements (hereinafter, referred to as “light-emitting elements”) have attracted many scientists.
In particular, image display devices are made up of a plurality of pixels each including a light-emitting element that emits light corresponding to a predetermined current value. Each pixel includes a thin film transistor (TFT) that controls the luminance of the light-emitting element. The TFT is made from, for example, amorphous silicon or polysilicon.
Long-time use of the TFT made from amorphous silicon (a-Si TFT) results in increase in the gate threshold voltage (hereinafter, referred to as “Vth”). The increase is called “Vth shift” of a-Si TFT. The rate of progression of Vth shift depends on application and operational conditions of the a-Si TFT.
For example, the progression of Vth shift of an a-Si TFT used as a switch as in a liquid-crystal display is slow because pulse current flows through the a-Si TFT for a given short time. On the other hand, the progression of Vth shift of an a-Si TFT used as a driving element for an organic light-emitting element as in an organic light-emitting image display panel is fast because high constant current flows through the a-Si TFT.
The Vth shift of a-Si TFT has two adverse effects on images. One effect is that different progression of Vth shift from one pixel to another deteriorates image uniformity. The other effect is that large Vth shift results in out of detection range of Vth and reduced pixel luminance.
On the other hand, known circuit technology is called Vth compensation (for example, Non-patent Document 1). According to this technology, a circuit configured to detect Vth shift of a-Si TFT and superimpose a video signal on the Vth shift obtains uniform images independently of the variation of Vth. It is known that performing Vth compensation can reduce the effects of Vth by a factor of about 5 to 10.    Non-patent Document 1: S. Ono et al., Proceedings of IDW '03, 255 (2003)